Droid X SD card issues

Okay, two issues/questions regarding my 16GB SD card:
- yesterday it was saying that it was full. So I deleted a bunch of apps and data and it said that there was now 650MB available. Today it says that there is 250MB free. WTF? Where did the 400MB go?
- I have lots of space available on the phone - several GBs. I see lots of stuff about moving things from the phone to the SD card, but I don't see any posts about how to move apps to the phone from the card, or to have an app store its data on the phone instead of the SD card. Can that be done?

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